I was looking for someone with a WdC anniversary this month, and came across your name on the list. When I saw the title, I got curious and had to click on this poem!
What a lovely poem for a beautiful couple. It not only speaks of their love for each other, but it also says a lot about your relationship with your sister and brother-in-law, and how observant you are of the big things and the little things that matter.
The verse structure and rhyme scheme work very well. The feeling you've created is of warmth and joy, with a little touch of humor. Great stuff! Did you read this out at the wedding? How was it received?
I have no big suggestions - just thought you might want to add color to the text.